A memory system that uses a nonvolatile memory such as a NAND flash memory as a storage device has a single level cell (SLC)-mode in which binary data (1-bit data) is written in a memory cell of the nonvolatile memory and a multi level cell (MLC)-mode in which multivalued data (data of 2 or more bits) is written in a memory cell of the nonvolatile memory.
The SLC-mode is preferable to write data in the nonvolatile memory at a high speed. However, the number of free blocks as a free space speedily decreases during the writing in the SLC-mode. For this reason, when data is continuously written, a compaction readily occurs during the writing and leads to a decrease in the write speed.
On the other hand, during the writing in the MLC-mode, the number of free blocks moderately decreases. For this reason, even if data is continuously written, the compaction rarely occurs during the writing, and the write speed is stable. However, the write speed in the MLC-mode is lower than that in the SLC-mode without the compaction, as a matter of course.
Hence, the performance of the memory system is influenced by whether the SLC-mode and the MLC-mode are appropriately selected.